The present invention relates generally to X-ray microanalysis, and more specifically, to the application of X-ray analysis to measure film stack characteristics on semiconductor devices.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for integrating circuits into semiconductor materials. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the semiconductor manufacturing process is prone to processing defects. Testing procedures are therefore critical to maintain quality control. Since the testing procedures are an integral and significant part of the manufacturing process, the semiconductor industry constantly seeks more accurate and efficient testing procedures.
A critical aspect of semiconductor fabrication involves the formation of the multiple conductive layers and liner layers. Each conductive layer includes the metal traces which form the paths along which electronic signals travel within semiconductor devices. Each of the conductive layers, are separated by a dielectric material layer and a liner layer. The dielectric material layer, commonly silicon dioxide, provides electrical insulation between the conductive layers. Portions of each conductive layer are connected to portions of other conductive layers by electrical pathways called xe2x80x9cplugs.xe2x80x9d The liner layers are formed between each conductive layer and each dielectric material layer to prevent the conductive material from diffusing into the dielectric material layer. The liner layer inhibits a conductive layer from diffusing into an underlying dielectric and short circuiting with an adjacent conductive layer. Of course, such short circuit formations are likely to be detrimental to semiconductor performance. In particular note, copper, a common conductive material used in semiconductor devices, diffuses very aggressively into silicon dioxide. The thickness and composition of the conductive and liner layers must be formed under extremely small margins of error. Thus, systems capable of testing the characteristics of these layers are very important.
Some of the current methods for measuring film stack characteristics include a four-point probe test system, eddy current testing, X-ray fluorescence testing, photo-induced surface acoustic wave testing, and X-ray micrography techniques using a single energy dispersive detector (EDS) and/or a single wave length dispersive detector (WDS).
Unfortunately, each of these methods have associated disadvantages which limit their usefulness. For example, the four-point probe test system requires the destruction of the specimens. The eddy current test has difficulty resolving film stack thickness and also has a spot size which is relatively large as compared to areas of interest on a semiconductor device. X-ray fluorescence testing is also limited by a large spot size. X-ray fluorescence also has difficulty distinguishing different film stacks thickness, generally results in poor measurements, and is a time consuming process. It is important for the testing speeds to keep pace with the increasing fabrication speeds so that the goals of maximizing manufacturing throughput may be achieved. A specific problem with photo-induced surface acoustic wave methods is that it has difficulty resolving the thickness of copper layers; copper being a common conductor used in semiconductors. In addition to their specific problems, the above mentioned methods generally cannot accurately measure characteristics of a film stack which has multiple layers.
An EDS system is described in U.S. Pat. No. 4,675,889 by Wood et al., which is incorporated herein in its entirety. EDS systems are typically capable of collecting and counting X-ray photons for specific wide ranges of energy. Specific materials will be expected to have specific peak photon counts at specific energy levels. Although an EDS system may be adequate for measuring a single thin film, this system does not work well for measuring multiple films (e.g., such as a conductive layer and an underlying liner layer). Since EDS systems have relatively low associated signal to noise ratios, some or all of the peaks are not measured accurately or not detected at all. For example, when the number of X-ray photons for a particular material type are expected to peak at two energy levels that differ from each other by a relatively small amount, the two peaks are likely to be collected together within an energy range that includes both peak energy levels. Additionally, if one of the peaks is significantly smaller than the other peak, only the larger peak will be detected. A system utilizing a single WDS detector detects X-ray photons having a specific energy level. A drawback to the single WDS system is that the single WDS detector must be reconfigured each time it is desired to detect X-ray photons having a different energy level. This slows the testing process for a film stack of an integrated circuit since the WDS must be reconfigured for each sample type, and if desired, for each of the various characteristic emission levels for each sample.
Currently there is no satisfactory method capable of testing a pattern semiconductor wafer in a non-destructive manner, with a high degree of accuracy, having a high throughput, and with a relatively small spot size. In order for the semiconductor fabrication industry to achieve higher goals of manufacturing throughput, a system capable of testing film stacks having the above listed traits would be desirable.
Accordingly, the present invention provides a non-destructive semiconductor testing system capable of efficiently measuring the composition and/or thickness of one or more layers within a film stack with a high degree of accuracy. In general terms, the present invention includes mechanisms for inducing X-ray emissions from a sample under test. The sample under test may have multiple film layers, such as conductor, insulator, and liner layers. In a specific embodiment, a charged particle beam is used to induce X-rays to emanate from one or more films on a semiconductor device. The charged particle beam penetrates at least two film layers (e.g., one conductive layer and one liner layer) of the film stack so that X-rays are produced in these penetrated layers. At least a portion of the X-rays are detected using one or more X-ray detectors that each detect X-ray photons having one or more specific energy levels. The X-rays may then be analyzed to determine characteristics of the penetrated layers, such as composition and thickness.
In one embodiment, an apparatus for measuring film stack characteristics on an integrated circuit is disclosed. The apparatus includes an electron beam generator that is configurable to direct an electron beam towards the integrated circuit such that the electron beam penetrates at least a conductive film layer and a liner film layer of the integrated circuit. The electron beam thereby cause X-rays to emanate from the integrated circuit. The apparatus further includes X-ray detectors positioned above the integrated circuit so as to detect at least a portion of the X-rays emanating from the integrated circuit. Preferably, each of the multiple X-ray detectors are wavelength dispersive system (WDS) detectors. This embodiment appears to have a relatively high degree of accuracy (ie., 0.5%).
In another aspect, the invention pertains to a method for measuring film stack characteristics on a semiconductor device. An electron beam is directed towards the semiconductor device so that the electron beam penetrates at least a conductive film layer and a liner layer. At least a portion of the X-rays which are caused to emanate from the device are detected with multiple X-ray detectors which detect X-ray photons having a specific energy level. In a preferred embodiment, each of the multiple X-ray detectors are WDS detectors. A reflective surface within each WDS is positioned to focus X-rays of a predetermined energy level upon a sensor within each WDS. In other words, each WDS is configured to detect X-rays at a specific energy level. In yet another aspect, data obtained from the detected X-rays is collected and analyzed. The present invention also provides a computer-readable medium containing computer code for performing the above described method.
In alternative method and in computer-readable medium embodiments, at least one property of a film stack is determined using raw data (e.g., measured X-ray counts) associated with the film stack. A set of film stack characteristic values (e.g, thickness and/or compositions of a conductive and liner layer) are selected and used to generate predicted data (e.g., predicted X-ray counts at one or more specific energy levels) by solving equations which model the film stack. A new set of film stack characteristic values are selected when the difference between the predicted data and the raw data is larger than a certain margin of error. When the difference between the predicted data and the raw data is less than the margin of error, the selected characteristic values are believed to be a sufficiently accurate measurement of the actual characteristic values. In a specific embodiment, the raw and predicted data represent count values of X-rays at specific energy levels, and the film stack characteristic values represent a thickness and a composition value of the film layers in the film stack.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.